Remove Gin dependency

I created a symlink to version 5 of gin at my local to validate it using MR!5 of issue https://www.drupal.org/project/jsonapi_query_builder/issues/3529219.

Adding screenshot for your reference

Thanks for this. Not necessarily related to this MR, but we should look into the following while we're working on this:

1. We check to see if Gin is installed, but not whether it's the admin theme. Should we check to see if it's actually the admin theme?
2. What is the user experience if Gin _isn't_ installed? Is it completely broken? If so, we should change the severity level of the requirements check.
3. Should the requirements check run on install instead of runtime?

I propose we remove the dependency altogether. I've successfully tested with with Gin 4.x and 5.x, Claro, and even Olivero, and it works fine in all of them. (It's squished in Olivero, of course, but it still works; and if you're using Olivero for your admin you're probably already collecting rent money from the voices in your head.)

In fact, it works so well in Claro, most people would probably be hard-pressed to tell the difference. Since that's the case with the default admin theme in Core, I don't think it's even worth mentioning Gin, much less putting it in hook_requirements().

Assuming we all agree on that, we just need to make one other decision: do we completely remove the Gin requirement from composer.json or not? It would technically be a backward compatibility break, because someone could have the theme installed solely as a transitive Composer dependency (i.e., because they've required JSON:API Query Builder and it in turn requires Gin). If we just remove it from our requirements, it would suddenly disappear from that person's codebase when they update. But I don't know if we have any sort of BC commitment. So...

1. If we need to preserve backward-compatibility, we should just loosen the Composer requirement so that it also supports Gin 5.x (the existing MR).
2. If not, we should remove the requirement altogether (a new MR I've opened).

I've removed the hook_requirements() implementation in both MRs, so unless I'm missing something, I think we're ready to just choose between them.

Technical notes:

1. Tests are going to fail, and that's expected. They're already failing in HEAD, and they're being addressed in #3529219: Replace JSON:API Schema dependency with Open API JSON:API module.
2. There is a js/react-app/src/gin.css and corresponding HTML classes in the frontend code that seem to imply Gin support, but as far as I can tell, they're not actually specific to Gin (they create no implicit dependency), and they didn't cause any problems in any of the other themes I tested. There's nothing about them in the documentation or the commit log. I'm guessing the naming convention is just an artifact of AI oddities. I suggest we just leave them in there until and unless we a more rigorous audit/clean-up in the future.
3. I deleted the entire jsonapi_query_builder.install file in both MRs, because all it had was the hook implementation. That will probably cause a merge conflict with #3529219: Replace JSON:API Schema dependency with Open API JSON:API module, but I think there would've been one either way, and it will be trivial to resolve.

Moved target to 2.x branch

Committed https://git.drupalcode.org/project/jsonapi_query_builder/-/merge_requests/9 to the 2.0.x branch here https://git.drupalcode.org/project/jsonapi_query_builder/-/commit/b95755...

Thanks!